The role of back ressure in injection molding is very important, which directly affects the filling of the melt and the quality of the plastic part.

Proper back ressure can improve the quality of the product

1. It can compact the melt in the barrel, increase the density, and improve the stability of the injection volume, product weight and size;
2. The gas in the melt can be “squeezed out” to reduce the gas splash on the surface of the product, the internal bubbles, and improve the gloss uniformity;
3. Slow down the retreat speed of the screw, make the melt in the barrel fully plasticized, increase the mixing uniformity of toner, masterbatch and melt, and avoid the phenomenon of color mixing in the product;
4. Appropriately increase the back ressure, which can improve the shrinkage of the surface of the product and the glue transfer around the product;
5. It can increase the temperature of the melt, improve the plasticization quality of the melt, improve the fluidity of the melt when filling the mold, and there is no cold material pattern on the surface of the product.

The impact of high back ressure on products

1. The melt pressure at the front end of the barrel is too high, the material temperature is high, the viscosity decreases, the counterflow of the melt in the screw groove and the leakage flow between the barrel and the screw increases, which will reduce the plasticizing efficiency;
2. For plastics with poor thermal stability (such as PVC, POM, etc.) or colorants, thermal decomposition is caused by the increase of the temperature of the melt and the increase of the heating time in the barrel, or the degree of discoloration of the colorant increases, and the surface color and gloss of the product deteriorate;
3. The back ressure is too high, the screw retreats slowly, and the pre-plastic recycling time is long, which will increase the cycle time and lead to a decrease in production efficiency;
4. The back ressure is high, the melt pressure is high, and the nozzle is prone to melt salivation after injection, and the cold material in the nozzle flow channel will block the nozzle or the cold material spot will appear in the product when the glue is shot again;
5. In the injection molding process, due to excessive back ressure, the nozzle often leaks, which wastes raw materials and causes the heating ring near the nozzle to burn out;
6. The mechanical wear of the preplastic mechanism and the screw barrel increases.

The impact of low back ressure on products

1. When the back ressure is too low, the screw retreats too quickly, and the melt density flowing into the front end of the barrel is small (loose), and more air is trapped;
2. It will lead to poor plasticizing quality, unstable injection volume, and large changes in product weight and product size;
3. The surface of the product will appear shrinkage, gas flowers, cold material lines, uneven gloss and other undesirable phenomena;
4. Air bubbles are easy to appear inside the product, and the surrounding area of the product and bone position are easy to go and not get enough glue.

Guidance method for back ressure setting

The adjustment of injection molding back ressure should depend on the performance of raw materials, drying conditions, product structure and quality conditions, and the back ressure is generally adjusted at 3-15kg/cm2. When there is a little air splash, color mixing, shrinkage on the surface of the product and large changes in product size and weight, the back ressure can be appropriately increased. When the nozzle has glue leakage, salivation, overheating and decomposition of the melt, discoloration of the product and slow recyclement, it can be considered to reduce the back ressure appropriatel